Method and apparatus for measuring formation conductivities from within cased wellbores by combined measurement of casing current leakage and electromagnetic response

Abstract

A method for determining spatial distribution of resistivity of Earth formations surrounding a wellbore having a conductive pipe therein includes measuring resistivity of the Earth formations using measurements of current leakage along the pipe at selected axial positions. Electromagnetic properties of the Earth formations are measured from within the pipe. The measurements of electromagnetic properties correspond to a larger axial distance and to a greater lateral distance than the measurements of resistivity from current leakage. The current leakage and electromagnetic measurements are jointly inverted to obtain a model of the spatial distribution.

1. A method for determining spatial distribution of resistivity of Earth formations surrounding a wellbore having a conductive pipe therein, comprising:
 measuring resistivity of the Earth formations using measurements of current leakage along the pipe at selected axial positions; 
 measuring electromagnetic properties of the Earth formations from within the pipe, the measurements of electromagnetic properties corresponding to a larger axial distance and to a greater lateral distance than the measurements of resistivity from current leakage; 
 jointly inverting the current leakage and electromagnetic measurements to obtain a model of the spatial distribution; and 
 storing the model in a computer readable medium. 
 
   
   
     2. The method of  claim 1  wherein the measuring resistivity using measurements of current leakage comprises:
 conducting an electrical current between a first selected position in the wellbore through the conductive pipe to a second position along the pipe near the Earth's surface; 
 digitally sampling a voltage drop measured between a third and fourth selected positions along the pipe between the first and second selected positions; 
 conducting an electrical current between the first selected position and a fifth selected position near the Earth's surface away from the pipe; 
 repeating the digitally sampling the voltage drop between the third and fourth position; 
 determining resistivity of the Earth formation from the digital samples of voltage drop and 
 storing the resistivity in a computer readable medium. 
 
   
   
     3. The method of  claim 2  further comprising moving the first, the second and the third positions, and repeating both the digitally sampling and the repeated digitally sampling. 
   
   
     4. The method of  claim 2  further comprising conducting a focusing current from a selected position along the pipe, the focusing current configured to constrain flow of current from the first position to the fifth position along substantially laterally outward paths from the wellbore in the lateral vicinity of the wellbore. 
   
   
     5. The method of  claim 4  further comprising controlling a magnitude of the focusing current so that a voltage drop measured axially along the wellbore between selected positions remains substantially zero. 
   
   
     6. The method of  claim 2  further comprising controlling a duty cycle of the current flowing from the wellbore to the second and to the fifth positions near the Earth's surface to correspond to an apparent conductivity of Earth formations. 
   
   
     7. The method of  claim 2  wherein the digitally sampling is performed at a rate of at least about one thousand times a frequency of the current flowing from the first selected position so as to enable determination of transient effects. 
   
   
     8. The method of  claim 2  wherein the conducting an electrical current between the first selected position and the fourth position, and the conducting electrical current between the first position and the fifth position comprises switching polarity of a direct current. 
   
   
     9. The method of  claim 8  wherein a frequency of the switching is within a range of about 0.2 to 20 Hertz. 
   
   
     10. The method of  claim 8  wherein the switching is performed according to a pseudo random binary sequence. 
   
   
     11. The method of  claim 2  wherein the conducting an electrical current between the first selected position and the fourth position, and the conducting electrical current between the first position and the fifth position comprises generating alternating current. 
   
   
     12. The method of  claim 11  wherein a frequency of the alternating current is within a range of about 0.2 to 20 Hertz. 
   
   
     13. The method of  claim 2  further comprising selecting an axial distance between the second and third positions in response to an expected resistivity of the Earth formations. 
   
   
     14. The method of  claim 13  wherein the selecting an axial distance comprises generating an initial model of the Earth formations, estimating a resistivity of the Earth formations from the digitally sampled voltage measurements and selecting the axial distance based on differences between the initial model and the estimated resistivities. 
   
   
     15. The method of  claim 2  further comprising selecting an axial distance between the first position and at least one of the second and third positions in response to an expected resistivity of the Earth formations. 
   
   
     16. The method of  claim 15  wherein the selecting an axial distance comprises generating an initial model of the Earth formations, estimating a resistivity of the Earth formations from the digitally sampled voltage measurements and selecting the axial distance based on differences between the initial model and the estimated resistivities. 
   
   
     17. The method of  claim 15  further comprising passing a focusing current through the pipe at selected axial positions, the axial positions for passing the focusing current selected in response to at least one of the initial model and the differences between the initial model and the estimated resistivities, the focusing current having a flow path selected to constrain the electrical current flowing from the first position substantially to a predetermined geometry. 
   
   
     18. The method of  claim 2  wherein the electromagnetic measurements comprise time domain measurements. 
   
   
     19. The method of  claim 2  wherein the electromagnetic measurements comprise frequency domain measurements. 
   
   
     20. The method of  claim 1  further comprising measuring a nuclear radiation property of the Earth formations, deriving an inference of at least one of composition of, fluid content of and fractional volume of pore space of the Earth formations; and storing the inference in a computer readable medium. 
   
   
     21. The method of  claim 20  wherein the nuclear radiation property comprises macroscopic neutron capture cross section.